In convection-enhanced delivery of drugs in solution into brain parenchyma in particular, the expansion or edema of the tissue is one of the most important determinants of the distribution of the infusate. The extracellular volume fraction of tissue can rise from a nominal 0.2 to 0.7 or so. This very dramatic increase in the fraction of fluid containing extracellular space can increase the hydraulic conductivity by orders of magnitude, thus directing flow of fluid into such edematous spaces in favor of other channels which are much more restricted because of smaller relative extracellular volume.
It is thus important to predict which regions of tissue, in particular brain tissue, are expanded and by how much, for given flow rates of infusate. As important as predicting the volumes of the nervous system that would readily expand, is to actually predict those places that cannot expand due to anatomic or physiological reasons. Those places that cannot expand would likely act as a barrier to fluid flow since other regions around them may become more conducive to the fluid flow.